Note 1 - Summary of Significant Accounting Policies - Cumulative-effect Adjustments (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2018		Jun. 24, 2017	Jun. 30, 2018		Jun. 24, 2017	Dec. 31, 2017	Dec. 30, 2017
Deferred profit		$ 1,709	[1]		$ 1,709	[1]		$ 5,350	$ 6,608
Income taxes payable		3,905			3,905			2,360	2,159	[2]
Retained earnings		168,040	[1]		168,040	[1]		151,783	$ 150,726	[2]
Net sales		99,817		$ 93,866	194,967		$ 174,963
Income tax provision		2,468		1,078	4,595		2,436
Net income		$ 11,648		$ 10,430	$ 19,770		$ 17,193
Basic: (in dollars per share)		$ 0.40		$ 0.38	$ 0.69		$ 0.63
Diluted: (in dollars per share)		$ 0.39		$ 0.36	$ 0.67		$ 0.60
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Deferred profit	[1]	$ 5,884			$ 5,884
Retained earnings	[1]	164,733			164,733
Net sales		99,556			192,218
Income tax provision		2,213			4,096
Net income		$ 11,642			$ 17,520
Basic: (in dollars per share)		$ 0.40			$ 0.61
Diluted: (in dollars per share)		$ 0.39			$ 0.59
Accounting Standards Update 2014-09 [Member]
Deferred profit								(1,258)
Income taxes payable								201
Retained earnings								$ 1,057
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Deferred profit	[1]	$ (4,175)			$ (4,175)
Retained earnings	[1]	3,307			3,307
Net sales		261			2,749
Income tax provision		255			499
Net income		$ 6			$ 2,250
Basic: (in dollars per share)		$ 0			$ 0.08
Diluted: (in dollars per share)		$ 0			$ 0.08

[1] Balance sheet line items include the cumulative-effect adjustment recorded on December 31, 2017.

[2] Derived from December 30, 2017 audited financial statements